The invention relates to unsaturated polyester resins having a low volume shrinking property upon curing.
Unsaturated polyester resins have a high shrinkage rate upon curing, amounting to 7-11% of volume shrinkage.
Molded articles that an unsaturated polyester resin, admixed and blended with a glass fibre or a filler such as calcium carbonate, are press-molded or injection-molded tend to cause curvature due to distortion, cracks, sink marks or blooming of the glass fibre. Such phenomena are often ascribed to the shrinkage of unsaturated polyester resin upon curing.
In order to solve the problems, there has heretofore been suggested to decrease the shrinkage of resin compositions upon curing by using a highly unsaturated polyester resin in combination with a thermoplastic resin such as vinyl polymers, e.g. polystyrene or polymethyl methacrylate, or saturated polyester resins, as disclosed in Japanese Patent Published Specification No. 46-41709. It is certainly possible to decrease the shrinkage rate of shaped articles by using a highly unsaturated polyester resin and by exerting fully the shrinkage-decreasing effect of a thermoplastic resin. On the other hand, however, the strength of shaped articles decreases generally by incorporation of a thermoplastic resin, down to about 70-80% when compared with ones containing no plastic resin. Further, the strength of unsaturated polyester resin itself, as well as its tenacy, decrease due to the fact that a highly unsaturated resin, in which more than 70 molar percent of acid component are unsaturated acid like maleic anhydride, has to be used.